U.S. Pat. No. 5,923,660 discloses a switched Ethernet data network. Such a data network is based on what is known as a network switch, which can link various stations of the data network to one another by point-to-point connections. The communication on the data network takes place by means of data packets. A data packet can be sent to just one station, to a number of stations or to all the stations, of the data network, the term “broadcast” being used in the latter case.
A switched data network is generally characterized in that one station can reach all the other stations of the switched data network only indirectly by appropriately transferring the data to be transmitted by means of one or more switching units. The individual connections required for setting up a respective communication connection between, in each case, two stations of the data network are referred to as point-to-point connections.
Data networks make communication between a plurality of stations possible by networking, i.e., connection, of the individual stations to one another. Communication here means the transmission of data between the stations. The data to be transmitted is sent as data telegrams, i.e. the data is packed together to form a number of packets and is sent in this form to the respective receiver via the data network. The term data packets is therefore also used. The term transmission of data is used hereinbelow synonymously with the above-mentioned transmission of data telegrams or data packets.
The networking itself is achieved, for example, in switched high-performance data networks, in particular the Ethernet, by at least one switching unit being switched in each case between two stations and being connected to both stations. Each switching unit can be connected to more than two stations. Each station is connected to at least one switching unit, but not directly to another station. Stations are, for example, computers, stored-program controllers (SPS) or other machines which exchange electronic data with other machines, and in particular process such electronic data.
In distributed automation systems, for example in the field of drive engineering, specific data must arrive at specific times at the stations intended for said data and be processed by the receiving parties. The term used here is real-time-critical data or data traffic, since, if the data does not arrive at the right time at the intended destination, there are undesired results at the station. According to IEC 61491, EN61491 SERCOS interface—basic technical description (http://www.sercos.de/deutsch/index deutsch.htm), successful real-time-critical data traffic of the type mentioned can be ensured in distributed automation systems.
It is also known from the prior art to use a synchronous, clocked communication system with equidistance properties in such an automation system. This is understood to mean a system comprising at least two stations which are connected to one another via a data network for data to be exchanged or transmitted between them.
The exchange of data takes place cyclically in equidistant communication cycles which are predefined by the communication clock used by the system. Stations are, for example, central automation units, programming, planning and design or operator control units, peripherals such as input/output modules, drives, actuators, sensors, for example, stored-program controllers (SPS) or other control units, computers or machines which exchange electronic data with other machines, and in particular process data from other machines. Control units are understood to mean regulating units, drives or control units of all types. Bus systems such as Fieldbus, Profibus, Ethernet, Industrial Ethernet, FireWire or else PC-internal bus systems (PCI) etc. are used as data networks, for example.
Nowadays, automation components (for example controllers, drives, etc.), generally have an interface with a cyclically clocked communication system. An execution level of the automation component (Fast-cycle) (for example position control in a controller, rotational-speed control, torque control of a drive) is synchronized with the communication cycle. This defines the communication clock. Other, low-performance algorithms (Slow-cycle) (for example temperature controls) of the automation component can also communicate with other components (for example binary switches for ventilators, pumps, etc.), only by means of this communication clock although a slower cycle would be sufficient. Using only one communication clock for transmitting all the information in the system results in stringent requirements being placed on the bandwidth of the transmission link.
System components known from the prior art use only one communication system or one communication cycle (Fast-cycle) for the communication for each process level or automation level, all the relevant information being transmitted in the clock of said communication cycle. Data which is required only in the slow cycle can be transmitted in a sequenced fashion, for example by means of additional protocols, in order to restrict the requirements made of the bandwidth. This means additional software expenditure in the automation components. Furthermore, both the bus bandwidth and the minimum possible communication cycle in the entire system are determined by the component with the lowest performance.
DE 100 58 524.8 discloses a system and method for the parallel transmission of real-time-critical and non-real-time-critical data. Such a system has means for transmitting data in at least one transmission cycle with an adjustable period, each transmission cycle being divided into at least one first part for the transmission of real-time-critical data for real-time control, and at least one second part for the transmission of non-real-time-critical data.
For an application of such a system it is assumed that an open, Internetbased communication is a spontaneous communication, that is to say that both the time of such communication and the quantity of data that is generated and transferred cannot be determined in advance. As a result, collisions on the transmission lines in bus systems or in the switching units in switched high-speed networks, in particular Fast Ethernet or Switched Ethernet, cannot be ruled out.
So that the advantages of Internet communication technology can also be used in real-time communication in switched data networks in the field of automation engineering, in particular of drive engineering, a mixed operating mode of real-time communication with otherwise spontaneous, non-real-time-critical communication, in particular Internet communication, is desirable. This becomes possible by virtue of the fact that the real-time communication which occurs predominantly cyclically in the applications under consideration, and can thus be planned in advance, is strictly separated from the non-real-time-critical communication, in particular the open Internet-based communication, which, in contrast, cannot be planned.
The communication between the stations takes place in transmission cycles, each transmission cycle being divided into at least one first part for the transmission of real-time-critical data for real-time control, for example of the industrial equipment which is provided for this purpose, and at least one second part for the transmission of non-real-time-critical data, for example in the case of the open, Internet-capable communication. A particularly advantageous embodiment of such a system is characterized by the fact that each station is assigned a switching unit which is provided for transmitting and/or receiving and/or forwarding the data to be transmitted.
An advantageous embodiment of such a system is characterized by the fact that all the stations and switching units of the switched data network always have a common synchronous time base as a result of synchronization with one another. This is a precondition for separation of plannable real-time communication from non-plannable non-real-time-critical communication. The separation of plannable real-time communication and of non-plannable non-real-time-critical communication is ensured by applying the method for synchronization according to the non-prepublished application DE 10004425.5.
By permanently applying this method even in the ongoing operation of a distributed automation system, all the stations and switching units of the switched data network are always synchronized with a common time base, which consequently means that all the stations and switching units have the same starting point and the same length of each transmission cycle.
Since all the real-time-critical data transmissions are known before the actual data transmission as a result of the cyclical operation and can therefore be planned in advance, it is ensured that real-time communication can be controlled for all the stations and switching units in such a way that faults, for example collisions, do not occur in the transmission of data of the real-time-critical data telegrams themselves, and all the planned critical data transfer times are complied with precisely.
A further particularly advantageous embodiment of such a system is characterized by the fact that all the non-real-time-critical data which is intended to be transmitted during the part of a transmission cycle which is provided for the real-time-critical communication is buffered by the respective switching unit and is transmitted during the part of this transmission cycle, or of a following transmission cycle, which is provided for the non-real-time-critical communication. Accordingly, unplanned Internet communication which possibly occurs in the first part of a transmission cycle which is reserved for the real-time communication is displaced into the second part of the transmission cycle, which is reserved for the spontaneous, non-real-time-critical communication, as a result of which disruption to the real-time communication is completely avoided.
The respective data of the spontaneous, non-real-time critical communication is buffered by the respective switching unit in question and only transmitted in the second part of the transmission cycle, which is reserved for the spontaneous, non-real-time-critical communication, after the part for the real-time communication has expired. This second part, i.e. the entire period up to the end of the transmission cycle, is available to all the stations for the unplannable, non-real-time-critical communication, in particular Internet communication, likewise without influencing the real-time communication because the latter is carried out with separate timing.
Collisions with the real-time-critical data telegrams in the switching units can be avoided by virtue of the fact that all the non-real-time-critical data which cannot be transmitted during the part of a transmission cycle which is provided for the transmission of the non-real-time-critical data is buffered by the respective switching unit and transmitted during the part of a later transmission cycle which is provided for the transmission of the non-real-time-critical data.
A further advantageous embodiment of such a system is characterized in that, for all the real-time-critical data telegrams to be transmitted, transmission and reception times are noted by sensors and/or receivers, and all the times for the forwarding of the real-time-critical data telegrams and the respectively associated connecting links via which the real-time-critical data telegrams are forwarded, are noted in all the respectively involved switching units, before the start of the respective execution of the data transmission, i.e. a note of when and to which output port a real-time-critical data telegram which arrives at the time X is to be forwarded is noted in a switching unit.
A further advantageous embodiment of such a system is characterized by the fact that the forwarding times are planned in such a way that each real-time-critical data telegram arrives at the latest at the forwarding time, or arrives earlier at the respective switching unit, but in all cases is not transmitted onward until the forwarding time.
In this way, the problem of imprecise timing, which is manifest in particular in the case of long transmission chains, is eliminated. As a result, the real-time-critical data telegrams can be transmitted or forwarded immediately, without an interval, i.e. less satisfactory use of the bandwidth in the case of real-time data packets is avoided. Of course, it is, however, also possible when necessary to insert transmission pauses between the transmission of the individual data packets.
A further advantage of the time-based forwarding is that the finding of destinations in the switching unit is no longer address-based because the port to which the data is to be forwarded is apparent from the outset. The optimum use of all the existing connecting links within the switched data network is thus possible. Redundant connecting links of the switched data network which cannot be used for the address-based switching through of the non-real-time-critical communication, because circularities of data packets would otherwise occur, can, however, be taken into account in advance in the planning of the forwarding paths and thus used for the real-time communication.
This permits redundant network topologies, for example rings for fault-tolerant real-time systems, to be implemented. Data packets can be transmitted redundantly on disjunctive paths and circularities of data packets do not occur. A further advantage of planned forwarding is that it is thus possible to monitor each part of a path without acknowledge messages and fault diagnosis can thus easily be carried out.
A further advantageous embodiment of such a system is characterized by the fact that a switching unit has two separate access points to the respective station, one access point being provided for exchanging real-time-critical data and the other access point for exchanging non-real-time-critical data.
This has the advantage that real-time-critical data and non-real-time-critical data are processed separately. The access point for the non-real-time-critical data corresponds to the commercially available interface of a standard Ethernet controller, as a result of which the existing software, in particular drivers, can be used without restriction. The same applies to the existing software for a non-real-time-capable data network.